Generally, a charge-coupled device (CCD) is used in a technical field relating an image sensor, a delay of an analog signal, and a shift register for a digital signal, for example. In such a charge-coupled device, plural electrodes are arranged on a surface of a device formation layer made of a semiconductor. Electric charges are stored in a potential well formed within the device formation layer by applying a control voltage to the electrode. Further, a position and a depth of the potential well can be changed by controlling the control voltages applied to the juxtaposed electrodes. Consequently, it is possible to transfer electric charges from one of the adjacent potential wells to the other.
Further, there has been proposed a technique of enabling sensitivity control of a light receiving element by applying such a charge-coupled device to the light receiving element. For example, document 1 (JP 2004-309310 A) discloses a light receiving element which has the same structure as an MIS element in which an electrode is arranged on an electrically insulating layer on a device formation layer constituted by a semiconductor layer doped with impurities. The electrode and the electrically insulating layer are made of light transmissive materials. When light strikes the device formation layer through the electrode and the electrically insulating layer, electric charges occurs inside the device formation layer.
In addition, the single light receiving element has the plural electrodes. With controlling control voltages applied to the electrodes, the area of the potential well within the surface of the device formation layer can be controlled. For example, the device formation layer has a conductivity type of n-type, and the electric charges are electrons. In this example, when the positive control voltage is applied to the electrode, the potential well accumulating electrons is formed at a part of the device formation layer associated with the electrode.
When the device formation layer receives light while the positive control voltage is applied to the electrode to form the potential well within the device formation layer, part of electrons generated close to the potential well is captured by the potential well and then accumulated in the potential well. The remaining electrons which are not accumulated in the potential well vanish at a deep part of the device formation layer due to recombination.
As mentioned in the above, the electrons are accumulated in the potential well. With changing the area of the potential well within the surface of the device formation layer, it is possible to vary a proportion of the electrons accumulated in the potential well to the electrons generated by light irradiation. In brief, the sensitivity of the light receiving element can be adjusted by varying the area of the potential well.
To control the area of the potential well with regard to the single light receiving element, it is sufficient that the number of the electrodes receiving the control voltages is adjusted. For example, when the single light receiving element has the five electrodes, the sensitivity is maximized by applying the positive control voltages to the respective five electrodes, and the sensitivity is minimized by applying the positive control voltages to no electrodes (the electrons are not accumulated).
In the light receiving element having the aforementioned structure, control of the control voltages to the respective electrodes causes a change in the position and the depth of the potential well. Thus, this light receiving element also functions as the charge-coupled device. In brief, this light receiving element can transfer the electrons (electric charges) accumulated in the potential well. Document 1 discloses a technique of applying the control voltages to the respective electrodes to accumulate electric charges corresponding to an amount of received light and subsequently applying the control voltage to the single electrode to store the electric charges in the potential well. In brief, a period for accumulating electric charges and a period for holding the accumulated electric charges are provided. The numbers of the electrodes to which the control voltages are applied are different in the respective periods.
Further, in a similar manner as the charge-coupled device, electric charges held in the potential well are moved between the potential wells formed adjacent to each other by controlling the control voltages applied to the respective electrodes, and finally are taken out from the light receiving element.
Besides, timings of changing the control voltages applied to the respective electrodes are synchronized with a clock signal used for taking out electric charges from the light receiving element. In brief, a time interval between the timings of changing the control voltages applied to the respective electrodes is an integral multiple of a period of the clock signal.
In the following explanation, it is assumed that electric charges are accumulated in a potential well formed by use of plural electrodes arranged in series and subsequently the accumulated electric charges are held in a potential well formed by use of one electrode.
For example, as shown in FIG. 4(b), the electric charges 12 are accumulated in the potential well 11 associated with the six electrodes 10A to 10F. Thereafter, as shown in FIG. 4(g), the electric charges 12 are stored in the potential well 11 associated with the single electrode 10A. FIG. 4(a) indicates positions of the respective electrodes 10. The respective electrodes 10 are distinguished by use of the reference numerals 10A to 10J. The ten electrodes 10 are used as one unit.
Further, in the instance shown in FIG. 4, the following operation is also performed. In this operation, as shown in FIG. 4(h), the electric charges 12 are accumulated in the potential well 11 associated with the six electrodes 10C to 10H, and thereafter, as shown in FIG. 4(g), the electric charges 12 are stored in the potential well 11 associated with the single electrode 10H.
The operation illustrated by FIG. 4(b) to (g) and the operation illustrated by FIG. 4(h) to (i) are the same except directions of movement of the electric charges 12 during transition from a charge accumulation state to a charge holding state are reverse to each other. In brief, these operations are symmetric or complementary to each other.
The following explanation is mainly made to the eight electrodes 10A to 10H of one unit of the electrodes 10 as illustrated in FIG. 4. As mentioned in the above, voltages applied to the respective electrodes 10A to 10H are switched to the control voltages at timings synchronized with the clock signal. Therefore, the control voltages are applied at timings as illustrated in FIG. 13. FIG. 13(a) shows an operation of a counter of counting the clock signals. FIG. 13(b) illustrates timings indicative of the light projection and the no light projection from a light emitting source to the target space. FIG. 13(c) to (j) show respective voltage variations of the electrodes 10A to 10H. The voltage has two levels, that is, a state (upper levee) in which the control voltage is applied to the electrode 10, and a state (lower level) in which no control voltage is applied to the electrode 10.
As apparent from FIG. 13, the five clock signals are necessary for a transition period T2 in which the electric charges 12 accumulated in the potential well 11 associated with the six electrodes 10A to 10F are moved to the potential well 11 associated with the single electrode 10A. When a unit period has the same length as the eightfold period of the clock signal and a period for holding the electric charges 12 has a length identical to the two periods of the clock signal, an accumulation period for accumulating the electric charges 12 has a length identical to the two periods of the clock signal. Thus, the length of the accumulation periods is one fourth of the unit period.
The electric charges 12 are newly accumulated during a period in which the electric charges 12 accumulated in the potential well 11 associated with the six electrodes 10A to 10F are moved to the potential well 11 associated with the single electrode 10A. However, in this period, it is impossible to accumulate electric charges 12 at high sensitivity. The transition period T2 is constant unless the number of the electrodes 10 is changed. Therefore, a relative proportion of the accumulation period is increased with an increase in the unit period. However, the increase in the unit period may cause such a problem that response performance of the light receiving element is deteriorated.
In the above explanation is made to an instance where the charge-coupled device has a function of the light receiving element defined as the MIS element. However, also in an instance where the charge-coupled device has no function of the light receiving element, the same problem is likely to occur. In brief, with regard to an operation including a process of moving electric charges accumulated in the potential well formed at a region over the plural electrodes to the potential well formed at a region over the fewer electrodes, a proportion of the transition period to the unit period is relatively high. Thus, time necessary for completion of the whole operation may be prolonged with an increase in the transition period, and then an undesired delay may occur.